diff --git a/src/lib/Hydra/Helper/CatalystUtils.pm b/src/lib/Hydra/Helper/CatalystUtils.pm
index d36dd706..165a6413 100644
--- a/src/lib/Hydra/Helper/CatalystUtils.pm
+++ b/src/lib/Hydra/Helper/CatalystUtils.pm
@@ -123,10 +123,16 @@ sub getChannelData {
             # `--include-outputs' flag passed to `nix-store'.
             push @storePaths, $build->drvpath;
         } else {
-            push @storePaths, $build->outpath;
+	    push @storePaths, $build->outpath;
         }
         my $pkgName = $build->nixname . "-" . $build->system . "-" . $build->id;
         $c->stash->{nixPkgs}->{"${pkgName}.nixpkg"} = {build => $build, name => $pkgName};
+        # Put the system type in the manifest (for top-level paths) as
+	# a hint to the binary patch generator.  (It shouldn't try to
+	# generate patches between builds for different systems.)  It
+	# would be nice if Nix stored this info for every path but it
+	# doesn't.
+	$c->stash->{systemForPath}->{$build->outpath} = $build->system;
     };
 
     $c->stash->{storePaths} = [@storePaths];
diff --git a/src/lib/Hydra/View/NixManifest.pm b/src/lib/Hydra/View/NixManifest.pm
index 8a672084..94301119 100644
--- a/src/lib/Hydra/View/NixManifest.pm
+++ b/src/lib/Hydra/View/NixManifest.pm
@@ -33,6 +33,8 @@ sub process {
         
         my $url = $c->stash->{narBase} . "/" . $escaped;
 
+        my $system = $c->stash->{systemForPath}->{$path};
+
         $manifest .=
             "{\n" .
             "  StorePath: $path\n" .
@@ -41,6 +43,7 @@ sub process {
             "  NarURL: $url\n" .
             "  NarHash: $hash\n" .
             ($narSize != 0 ? "  NarSize: $narSize\n" : "") .
+            (defined $system ? "  System: $system\n" : "") .
             "}\n";
     }